Pocket sand glass, c 1500.

Image number: 10313718

This portable sand glass is enclosed in a brass case. The timekeeping depends on the regular flow of sand through a small orifice. Early sand glasses, such as this one, consisted of a pair of glass bulbs with their necks in contact with a metal plate into which a small hole had been drilled, and which were bound together by thread. By the middle of the 18th century they were blown in one piece, with a small hole through which the sand could be inserted. The time taken for one bulb of the sand glass to empty is a definite interval, at the end of which the glass can be turned over and the process repeated. Sand glasses are useful for measuring long periods of time, but prove inadequate for short time intervals.